The hook that should do the trick is Addon.prototype.postprocessTree. Now we have two choices, we can build a standalone addon, via ember addon or we can create a light-weight in-repo addon via ember g in-repo-addon. Typically for these types of situations, I prefer in-repo-addons as they don't require a second project, but otherwise they are the same.

ember g in-repo-addon remove

we need to install broccoli-stew via npm install --save broccoli-stew

include it var stew = require('broccoli-stew');

add hook postprocessTree to the add-on

when the postprocessTree is for the type we care about, use broccoli-stew to remove the directories we no longer care care.